1. Field of the Invention
The present invention relates to a reducing mill disposed at the rear of a plant for manufacture of electro-resistance-welded steel tubes wherein a narrow and long successive steel plate (a strip) is gradually formed into a cylindrical shape, seam-welded, and then, reduced with its diameter by the reducing mill.
2. Related Art
A previous reducing mill having three rolls (three-roll-type reducing mill) comprised a stand supporting three rolls exclusive to each product size. If the rolls have worn away, the stand supporting the rolls is replaced with another stand supporting new three rolls. The worn rolls are ground and the stand having the rolls is used for diametrically larger products.
Later, the three-roll-type reducing mill has been improved so that its stand is provided with mechanisms for adjusting the position of the roll to mill a formed electro-resistance-welded tube into various sizes, as disclosed in an earlier application (Japanese Patent Application Laid-Open No. 9-262620) filed by the present applicant.
For the purpose of prevention of an irregularity or a flaw on an outer surface of a finished tube caused by positional difference between adjacent rolls in a radial direction of the tube, it is important for adjustment of a caliber formed by the three forming rolls of the three-roll-type reducing mill corresponding to a reduced diameter of a steel tube that each of two positionally-adjustable forming rolls is moved along a tangent to a curve of a positionally-fixed forming roll adjacent thereto for abutting against an objective tube at each of opposite ends of the positionally-fixed forming roll, in other words, along a direction at a 30.degree. angle from a radial line of the positionally-fixed forming roll across the center of the objective tube.
This reducing mill comprises three rotary shafts; one is a drive shaft for one positionally-fixed main forming roll, and the other two are follower shafts for respective positionally-adjustable follower forming rolls which are driven by the drive shaft through respective bevel gears. Each rotary shaft is disposed in parallel to a shaft serving as an axis of a roll (a roll-axis shaft) on which each roll is fixed and drivingly connected with the roll-axis shaft through gears, so that the roll-axis shaft can be axially moved within the backlash between the gears. Also, the roll-axis shaft can be moved in perpendicular to its axis (radially) by sliding means. A movement of each of the follower forming rolls along a tangent to the curve of the main forming roll for abutting against an objective tube at each of the opposite ends of the main forming roll results from the two axial and radial movements of the roll-axis shaft.
However, since the axial movement of the roll-axis shaft and the radial movement thereof are independent of each other, each of the follower forming rolls, actually, cannot move straightly along the above-mentioned tangent. It is difficult to adjust the position of each of the follower forming rolls along the tangent exactly and for a short time by two operations for different movements of the roll-axis shaft of the follower forming roll. Furthermore, a proper positional adjustment of the follower forming rolls for the moment cannot be made during the processing of the reducing mill under inspection of finished products, even if any problem is found on the product finished by the reducing mill. It is only possible that a stand structure supporting the three forming rolls is adjusted with its two follower forming rolls in their positions before its incorporation into the plant for manufacture of electro-resistance-welded steel tubes or while the reducing mill or the entire plant is shut down in power for required adjustment thereof.